In earlier versions of UB, there was a feature which calculated the estimated size of the files selected with the file selection logic, but for a few major releases it disappeared.
It was very usefull when creating ad-hoc backups and find enough free space for the backup, or planning backup strategy to decide which backup should direct to which media.
I know, that it was a very slow action when opening the file set, but if there were an option (maybe there is, but I can not find yet) to enable or disable enumeration on load, I would like it very much :)
Or, if you can suggest third party softwares which can show me this information using the archive bit to filter, it would be an option, but then I have to select the necessary folders again and again independen from the set I will use for the backup.

This went away because it was requiring the interface to enumerate every file on disk. When partitions were smaller and had less files it wasn't a real issue, but we started getting a lot of complaints for those with partitions that had millions of small files because it would take 20-30 minutes or more just to enumerate the contents of the partition. Now we only enumerate the current level that we are looking at, and possibly one more level deep.

It would be a relatively easy script to create in vbs that would look at each file and if the archive bit was on, then get the file size and add it to the total.

Yes, the enumeration of the whole partition should be too much, and the dir command with the /ar cwitch can do it.
I am looking for a solution to combine it with the file selection list defined in the .ub file.
Sometimes it would be usefull for me to know the estimated size before the backup starts. When running a scheduled backup, it could check the size of the selected and modified files (when I enable this function), and compares to the free space on the target media.
Something like the Check if device is ready function in the sceduler options.
If the free space was not enough, it could send me an alert to free up some space on the NAS for example... (it should not be slower than filling the media, and restart the job)